When a distance runner begins to struggle early on in workouts and races, suffers from excessive fatigue, and often feels there's no "gas in the tank" in the final half of hard efforts, iron deficiency anemia should be one of the first problems to rule out. Having adequate iron stores is essential to any endurance sport, as your ability to run (or swim, or row, or ski, etc.) is predicated by your ability to get oxygen to your muscles, which is accomplished by your red blood cells. Iron deficiency anemia impedes the body's ability to manufacture red blood cells, and causes a marked decrease in performance. Red blood cells are comprised almost entirely of a protein called hemoglobin, and at the core of that protein is an iron atom. Oxygen binds to hemoglobin by binding with the iron atom at its center. And hence, if there isn't enough iron available to make red blood cells, there aren't enough red blood cells to carry oxygen to the muscles. And no oxygen means no high-level performance. Today I'm going to go in-depth on the issue of iron and iron-deficiency anemia, because it is often misunderstood, even within the medical profession. Technically, anemia only refers to low hemoglobin, but as we'll soon see, it's possible (and very common) to have low iron stores but not have low hemoglobin. There is mounting evidence that even iron deficiency without anemia is harmful to endurance performance. We're about to cover all of this in depth.
Introduction: the biology of red blood cells
As usual, though, we need a crash-course instructional first. This time, in blood physiology. As mentioned above, red blood cells are the oxygen-carriers of the bloodstream and are comprised mostly of hemoglobin, which in turn contains iron. Any old red blood cell circulating in the blood stream "lives" about three or four months before it's resorbed or destroyed. Your body always tries to keep enough red blood cells in reserve to meet the demands you put on it. So, when you go for several sessions of hard training, your body responds in turn by synthesizing more red blood cells. They are made in bone marrow, and their synthesis is stimulated by the hormone EPO. More red blood cells (generally) means better performance. Illegal dopers boost their red blood cell levels by injecting recombinant EPO; Alberto Salazar's runners boost their red blood cell levels by sleeping in altitude tents, and the Kenyans (unwittingly) boost their red blood cell levels by living and training at high altitude.